Project Summary/Abstract Broadly neutralizing antibodies (bNAbs) may be crucial component of the protective immunity conferred by an effective HIV-1 vaccine. Ab responses in natural HIV-1 infection are overwhelmingly non-neutralizing. However, a fraction of patients do eventually develop exceptionally broad and potent bNAb responses. We hypothesize that a better understanding of the specificities that underlie this broad serum neutralization will enable us to identify the most desirable targets and develop a full spectrum of vaccine design strategies. New epitopes may be particularly amenable to design or attractive by being unusually broad or potent. Researchers have recently accessed various monoclonal bNAbs (mbNAbs) by high throughput functional screening and/or by targeted selection of memory B cells from infected donors who exhibit broad serum NAb responses. However, these efforts remain challenging, in part due to the rarity of desirable memory B cells and the lack of authentic baits to select new specificities. Based on the premise that native Env trimers are the exclusive targets of NAbs, here we propose using virus-like particles (VLPs) bearing only native Env trimers as baits to retrieve novel mbNAbs that will illuminate novel sites of vulnerability. Our Specific Aims are: Specific Aim 1: To investigate monoclonal bNAb relationships on native trimer VLPs. We will determine the binding relationships of various mbNAb pairs by trimer VLP ELISA. Any antagonistic or synergistic combinations we identify could impact the therapeutic or prophylactic applications of mbNAb combinations. Selected mbMAb combinations will be further investigated in neutralization synergy assays. Specific Aim 2: To map the specificities of broadly neutralizing sera using native trimer VLPs. We will evaluate the ability of a panel of broadly neutralizing HIV+ sera to inhibit trimer VLP binding by the panel of mbNAbs used in Aim 1. This will allow us to prioritize B cell selections (Aim 3) from donor PBMCs whose sera exhibit novel NAb specificities. Specific Aim 3: To rescue mbNAbs using fluorescent trimer VLPs as baits. With an expert collaborator, we will develop methods to use fluorescently labeled trimer VLPs as baits and probe donor PBMCs to label and retrieve mbNAb clones responsible for broad serum neutralization. We will prioritize donors whose sera appear to target unique epitopes from mapping studies in Aim 2. Specific Aim 4: To characterize new mbNAbs. We will determine the specificity, breadth and potency of new mbNAbs. We will examine their binding relationships with other mbNAb specificities on the native trimer, as in Aim 1. To better understand their ontogeny, we will also examine their sequences, segment usage and divergence from nearest germline.